The present invention relates to a silicone pressure-sensitive adhesive composition. More particularly, the invention relates to a hot melt adhesive which cures upon exposure to ambient moisture.
Silicone pressure-sensitive adhesives (hereinafter also referred to as PSAs) typically contain at least two primary components, namely a linear siloxane polymer and a tackifier resin consisting essentially of triorganosiloxane (M) units (i.e., R.sub.3 SiO.sub.1/2 units, in which R denotes a monovalent organic group) and silicate (Q) units (i.e., SiO.sub.4/2 units). In addition to the above two ingredients, silicone PSA compositions are generally provided with some crosslinking means (e.g., peroxide or hydrosilylation cure systems) in order to optimize various properties of the final adhesive product. In view of the high viscosity imparted by the polymer component, these PSA compositions are typically dispersed in an organic solvent for ease of application.
A copending application Ser. No. 08/063,105 which is a continuation of U.S. patent application Ser. No. 07/748,478, filed on May 14, 1993 (EP Publication 529841A1) to Krahnke et al. describes silicone PSAs that cure in the presence of moisture to permanent adhesives. The compositions are particularly useful as structural adhesives for adhering glass panels to structural materials such as concrete, aluminum and steel. The two required ingredients of the PSA are an MQ resin and a high consistency polydiorganosiloxane gum. One or both ingredients of the PSA compositions described in this copending application contain moisture activated alkoxy curing groups. Due to the high viscosity of gum type polydiorganosiloxanes even at elevated temperatures, the compositions are not suitable for application as heated molten materials and are typically applied in solution form wherein the solvent is removed prior to curing of the composition or evaporates during the curing reaction.
Another moisture-curable PSA system is disclosed in Japanese laid open patent application (Kokai) No. 4(1992)/81,487. These PSAs comprise (1) 100 parts by weight of an MQ resin having a hydroxyl content of up to 0.7 weight percent, (2) a liquid polydiorganosiloxane containing hydrolyzable terminal groups and (3) a condensation catalyst to promote curing of the composition in the presence of moisture. The molar ratio of silanol groups in the resinous copolymer to hydrolyzable terminal groups in the liquid polydiorganosiloxane is from 1 to 10. A characterizing feature of these compositions is their ability to retain the characteristics of pressure sensitive adhesives under both dry and wet conditions following reaction of the moisture reactive groups.
U.S. Pat. No. 5,091,484 to Colas et al. describes elastomer-forming compositions containing (1) a hydroxyl- or alkoxy-terminated polydiorganosiloxane, (2) an alkoxy-functional MQ resin that is preferably liquid under ambient conditions and (3) a titanium-containing curing catalyst. These compositions are described as flowable under ambient conditions and cure in the presence of atmospheric moisture to yield elastomeric materials. The resins of Colas are produced by reacting a --SiH site on the resin with a alkenyl containing alkoxy silane in the presence of platinum.
Moisture-curable compositions which cure to elastomers are also disclosed by Favre et al. in U.S. Pat. No. 4,143,088. These compositions are prepared by mixing (a) a hydroxyl-terminated polydiorganosiloxane, (b) an MQ resin, (c) an alkoxylated organosilicon compound and (d) an organic titanium derivative. Before being cured, these systems are liquids under ordinary conditions.
Copending U.S. patent application Ser. No. 08/076,615 to Cifuentes et al., commonly owned, discloses a hot melt composition comprising (i) a solid hydroxyl-functional MQ resin, (b) a diorganopolysiloxane polymer containing at least two terminal alkoxy radicals, (iii) a hydrolyzable silane and (iv) a cure catalyst. These compositions have a high initial adhesive strength and cure in the presence of moisture to non-tacky elastomers.
It has now been found that the strength properties and adhesive strength at elevated temperatures can be improved by the use of an alkoxy/hydroxyl functional MQ resin wherein some or all of the hydroxyl groups on the MQ resin have been replaced with alkoxy functionality.
It is an object of the instant invention to show a moisture-curable pressure sensitive adhesive composition which has improved strength properties, adhesive strength at elevated temperature and storage stability.